1. Field of the Invention
The present disclosure pertains to the cooling of internal combustion engines and, more particularly, to a unique fin design for use with air-cooled engines, such as aircraft engines, automobile, truck, and motorcycle engines, and stationary, fan-cooled engines.
2. Description of the Related Art
In general, air-cooled engines have fins that require the cooling air to flow in a direction perpendicular to the axis of the cylinder. In a single cylinder engine, or an engine with a single bank of cylinders (such as a V2, a horizontally opposed 2, or a single bank radial), this is not a bad configuration. There is adequate space for fins and cooling air. Even a double bank radial works fairly well because the cylinders of the rear bank are oriented between the cylinders in the front bank, so there is adequate access for the cooling air to reach the aft cylinders. In the horizontally opposed air-cooled engines with 4 or more cylinders that are commonly used in automotive and aircraft applications, having the fins oriented perpendicular to the longitudinal axis of the cylinders is a serious disadvantage. The air flow must be oriented parallel to the fins. That means either (1) the air flow thru the fins must be parallel to the axis of the crankshaft, which provides less cooling for cylinders behind the front cylinder, or (2) it must be perpendicular to both the crank axis and the cylinder axis, which is the orientation used in all modern applications.
It takes power to drive cooling air thru the fin structure. Ultimately, in any mobile application, this power must come from the engine being cooled. This reduces the useful power output of the engine, and the net efficiency of the engine. There has been substantial effort for more than a century in designing efficient fin configurations for cooling engines with a minimum of lost power. To achieve efficient cooling, it is desirable to have the cross section of any given gap between fins to remain a constant area as the air passes thru the engine. With this configuration, the air moves with a constant velocity, and a minimum of power is required to provide a given amount of cooling. In addition, the path length thru the engine should be minimized to maintain a thin boundary layer between the fin and the moving air.
Now consider the situation in standard down-draft (or up-draft) cooling where essentially all the air must pass between the cylinders. The air enters above the cylinder and head, which are typically 10 to 20 cm wide. Then it passes thru the gap between the cylinders, typically 1 to 2 cm wide. Then it is blown out beneath the cylinders and heads, again 10 to 20 cm wide. With careful duct design, the cooling air can be guided around the engine to pass over most of the fins. But the restriction at the passage between cylinders always increases the pressure required to force sufficient air thru the engine, and there are unavoidable dead-air regions above and below the cylinder and combustion chamber where very little cooling occurs. The power required is the product of pressure times volume flow rate. The volume flow rate is fixed by the cooling requirements of the engine. If there is a restriction in the flow path that has half the area of the rest of the path, the flow velocity at that point will be twice as high as the velocity over the rest of the fin. Since pressure drop increases approximately with the square of flow velocity, the pressure drop per unit distance of air travel in the restriction will be four times as high as in the rest of the engine. With the air flow passing down between the cylinders, this is unavoidable. The result is excessive power required for cooling (very undesirable), and the possibility of insufficient cooling under some or all operating conditions (even more undesirable).
It does little good to try to cool the engine from the “top” (farther from the crank shaft). The rocker arms sit on top of the engine, and that assembly introduces so much thermal impedance that it is impractical to cool the heads by using fins over the rocker arms. Porsche has developed a head in which the two valves are one above the other, as opposed to side by side, giving more space for fins and air passages between the heads. This requires a tricky valve linkage and does nothing for the flow restriction between cylinders and the base of the heads.
For a specific example of present cooling problems, consider the Jabiru engine, built in Australia. The Jabiru has several desirable characteristics. It is a very compact engine for its power rating. Largely as a result of this, it is considerably lighter than other engines of similar power. Also, the small size makes the structure strong. Size and weight are important in many applications, and critical in aircraft. Strength is always desirable. A 6 cylinder Jabiru rated at 130 horsepower (100 kW) is essentially the same size as, and lighter than, a 4 cylinder Volkswagen producing half the power. There is no free lunch. The cost of the reduced size and weight of the Jabiru engine is that the compact design makes it essentially impossible to cool the engine when operated at rated power. Thru the remainder of this disclosure, the Jabiru engine will serve as the model. However, all the results from this analysis of the Jabiru engine are obviously applicable to other engines, including in-line and horizontally-opposed air-cooled engines.
FIG. 1 is a schematic representation of two adjacent cylinders 20, 22 of an engine, looking down an internal axial bore 24, 26 of each of the cylinders 20, 22. Each cylinder 20, 22 has a protrusion 27, 29 that inserts into a mating recess in the bottoms of the heads (not shown in this view). A plurality of fins 32, 34 surround the cylinders 20, 22. In addition, an engine needs head bolts (not shown), which are received in bolt holes 36. For strength, the head and cylinder both require a reasonable amount of material 38 surrounding the bolt holes 36. In the case of the Jabiru engine, there are six head bolts holding each head and cylinder together. This gives a great improvement in strength and rigidity over VW and Porsche engines which use only 4 head bolts. In many aircraft engines, the top of the cylinder is threaded and the entire head screws on. This gives even better mechanical stability than the Jabiru engine, but the added space required for the threads further restricts air flow between the edges of the heads.
Now consider the situation faced by the cooling air. The air typically enters at the top of the engine and flows down over the fins of the head and cylinder (downdraft cooling). The situation does not change much if the direction of flow is up from below the engine (updraft cooling). The air enters the fin structure in a region where the fins are typically 30 mm high and is squeezed between the cylinders where, in the case of the Jabiru engine, the fins are only 5 mm high. Thus, the air must travel 6 times as fast while it is between the cylinders, which requires 36 times as much pressure drop per unit distance traveled, and 36 times the power per unit distance of flow. Ultimately, this power comes from the engine, and this consumption of power decreases the power available to do useful work.
The problem is intensified in the Jabiru engine, where the use of six head bolts means that there is a long path length where the air must travel at high velocity. Also, when forcing air to flow around a cylindrical obstacle, the air flow tends to leave a dead air zone ahead of the center of the cylinder, and a much bigger dead air zone behind the center of the cylinder (ahead and behind from the perspective of the flowing air). Careful use of ducts to guide the air will reduce the sizes of these dead air regions, but it cannot eliminate them entirely. Another problem is that the conductivity of heat from the metal fin to the air increases with increasing air velocity. Where the air moves slowly, a thick boundary layer forms, and conductivity into the air is low. In the situation shown in FIG. 1, the air travels slowly over 90% of the fin area, giving poor cooling, and where the air travels rapidly; there is not enough fin area to give adequate cooling.
Now consider the path length of the flow thru typical fins. In round numbers, this path length will be π times the average radius of the cylinder fins. If the cylinder has a bore of 100 mm, that is a radius of 50 mm. The cylinder wall has a thickness of about 5 mm, and the head must surround that by about an additional 5 mm. Thus, the radius from the longitudinal axis of the cylinder to the base of the fins will be about 60 mm. If the fins are 30 mm high, the average radius of the fins becomes 75 mm. That gives a flow path length of 235 mm. This is a much longer path length than is desirable from purely thermodynamic considerations. Typical automotive radiators have path lengths of under 50 mm, and they usually have staggered fins within that distance. Aircraft oil coolers typically have air path lengths of 15 to 20 mm, with staggered fins within that distance. A flow path length of 235 mm is asking for thick boundary layers and unacceptable conductivity from the fin to the air.